[CVE-2026-72122][MODERATE REGULAR] can: bcm: fix lockless bound/ifindex race and silent RX_SETUP failure

[AL-KERNEL <alkernel@kernelcve.org>]

CVE: CVE-2026-72122
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: can: bcm: fix lockless bound/ifindex race and silent RX_SETUP failure
Commit: 9e60c586faeaed80d55ab8ce2a4b8e56133bc395
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9e60c586faeaed80d55ab8ce2a4b8e56133bc395
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72122
Analysis date: Sat, 15 Aug 2026 06:18:44 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Borderline, manual review recommended
Manual review required: YES

Summary:
A local race in CAN BCM can make bcm_sendmsg observe inconsistent bound and ifindex state, temporarily allowing a socket bound to one CAN interface to behave like an any-interface socket, with limited confidentiality or integrity impact and no demonstrated memory corruption.

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ABOUT THIS REPORT
======================================================================

The original Linux kernel CVE announcement for CVE-2026-72122 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72122

The original announcement does not normally provide a security severity
estimate, CVSS assessment, or enough information to determine whether the
reported kernel bug represents a practically relevant security issue.

This report was generated by AL-KERNEL, an AI-assisted Linux kernel
vulnerability analysis system developed by Alexander Larkin. It combines
an autonomous classifier with LLM-assisted technical analysis and a
separate ActionableScore mechanism.

The purpose of this report is to prioritize Linux kernel CVEs before
manual review, identify cases that require prompt investigation, and
support automatic closure of issues that are unlikely to have meaningful
security impact.

Published priority for this report: MODERATE REGULAR
Manual review required: YES

A detailed explanation of the methodology and priority rules is included
at the end of this message.

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AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-72122	MODERATE	CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:L';CWE-362;CWE-667;CWE-284;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:N';BEST CVSS score: '4.5';DESCR 'A race in CAN BCM can let bcm_sendmsg observe bo bound and bo ifindex in an inconsistent state while another path updates the same socket under lock_sock. This can temporarily turn a socket that was intended to be bound to one CAN interface into behavior similar to an any interface binding, which may expose or send CAN BCM traffic outside the expected interface scope. For the CVSS the PR:L is used because a local process must create and operate a CAN BCM socket and reliably race sendmsg with connect or device unregister related state changes. The issue is not directly reachable from an IP network and is triggered through local socket control paths. Impact is mainly a local interface scope bypass with limited confidentiality or integrity impact, and possible local availability impact for affected CAN applications rather than a proven kernel memory corruption primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline, manual review recommended (with actual score 4)	YES	DISK INIT RACE HARDWARE LINUS  INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
======================================================================

ActionableScore=4
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 4
* Final recommended bucket: **Borderline, manual review recommended**::

## 2. Signal breakdown

Triggered signals:

* Local unprivileged trigger: +1
  A local process can operate a CAN BCM socket if PF_CAN/CAN_BCM is available and policy allows socket creation.

* Cross-boundary isolation/policy bypass: +1
  The race can make a socket intended to be bound to one CAN interface behave like an any-interface binding. This may cross an intended local CAN interface scope boundary.

* Confidentiality impact plausible: +1
  Limited exposure of CAN BCM traffic outside the expected interface scope is plausible.

* Integrity impact plausible: +1
  Limited unintended sending or matching of CAN BCM operations across interface scope is plausible.

* Hard or unreliable race / special timing required: -1
  The bug depends on racing bcm_sendmsg() with bcm_notify(), bcm_connect(), or device unregister state changes.

Paranoid-only signal:

* Availability impact realistic: +1
  The silent RX_SETUP success and broken RX operation may cause local application-level availability impact, but there is no demonstrated reliable kernel crash.

Not triggered:

* No generic memory corruption signal.
  The patch does not show UAF, OOB write, double free, stale callback, refcount takeover, or type confusion.

* No privilege escalation signal.
  The visible primitive is interface-scope confusion, not a kernel memory corruption or credential/capability takeover primitive.

* No remote reachable signal.
  This is not directly reachable from an IP network. It requires local CAN socket operations.

## 3. Reachability analysis

The likely trigger is a local process using CAN BCM sockets and racing sendmsg against connect or device unregister related state changes. Namespaces may matter if an untrusted user can access CAN sockets or CAN devices inside a delegated network namespace, but creating or managing CAN/vcan interfaces normally requires CAP_NET_ADMIN. The affected path is not common on general-purpose servers, but it is reachable through ordinary local socket APIs on systems with SocketCAN/CAN BCM enabled and usable CAN interfaces. Call-site confidence: high, because the relevant bcm_sendmsg(), bcm_notify(), bcm_connect(), bcm_release(), and bcm_rx_setup() paths are visible in the patch.

## 4. Severity interpretation

This behaves like a borderline actionable Moderate rather than an Important-class kernel vulnerability. The theoretical security impact is a local policy or interface-scope bypass between CAN interfaces with limited confidentiality and integrity impact. Realistic exploitation evidence is weaker because the primitive is race-dependent and there is no demonstrated memory corruption, kernel crash, privilege escalation, or remote network trigger. The issue should not be treated as a high-end kernel memory-corruption CVE based on the supplied patch.

## 5. One-sentence report phrase

A local race in CAN BCM can make bcm_sendmsg observe inconsistent bound and ifindex state, temporarily allowing a socket bound to one CAN interface to behave like an any-interface socket, with limited confidentiality or integrity impact and no demonstrated memory corruption.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Manual review is recommended because this is a race in a networking subsystem and the real impact depends on local CAN access policy, namespace delegation, and whether multiple CAN interfaces represent different trust domains.

======================================================================
UPSTREAM PATCH SUMMARY
======================================================================

Patch: can: bcm: fix lockless bound/ifindex race and silent RX_SETUP failure
Commit: 9e60c586faeaed80d55ab8ce2a4b8e56133bc395
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9e60c586faeaed80d55ab8ce2a4b8e56133bc395

Commit description:

bcm_sendmsg() reads bo->ifindex and checks bo->bound before taking
lock_sock(), while bcm_notify(), bcm_connect() and bcm_release() all
mutate both fields under that same lock. Because the lockless reads
and the locked writes are unordered with respect to each other, a
racing bcm_notify() (device unregister) or bcm_connect() (concurrent
bind on another thread sharing the socket) can make bcm_sendmsg()
observe an inconsistent combination, e.g. a stale bound=1 together
with the now-cleared ifindex=0, silently turning a socket bound to a
specific CAN interface into one that also matches "any" interface.

Keep the lockless bo->bound check purely as a fast-path reject, and
move the ifindex read (and a bo->bound re-check) into the locked
section, where every writer already serializes. This removes the
possibility of observing the two fields torn against each other,
rather than trying to fix it with more READ_ONCE()/WRITE_ONCE() pairs
on two independently updated fields. Annotate the now-purely-lockless
bo->bound accesses consistently across all its write sites.

Also fix bcm_rx_setup() silently returning success when the target
device disappears concurrently instead of reporting -ENODEV, so a
broken RX op is no longer left registered as if it had succeeded.

Fixes: ffd980f ("[CAN]: Add broadcast manager (bcm) protocol")
Reported-by: Ginger <ginger.jzllee@gmail.com>
Closes: https://lore.kernel.org/linux-can/CAGp+u1aBK8QVjsvAxM2Ldzep4rEbsP9x_pV3At4g=h1kVEtyhA@mail.gmail.com/
Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
Link: https://patch.msgid.link/20260714-bcm_fixes-v15-2-562f7e3e42da@hartkopp.net
Cc: stable@kernel.org
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/can/bcm.c

Diff excerpt:

Not included in this email. See the upstream URL for the full patch.

Full patch:
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9e60c586faeaed80d55ab8ce2a4b8e56133bc395

======================================================================
DETAILED REPORT METHODOLOGY
======================================================================

The original Linux kernel CVE announcement for CVE-2026-72122 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72122

The original CVE announcement normally does not include a security-level
estimate. In particular, it may not contain a CVSS assessment, an impact
level, or enough information to determine whether the reported bug is a
practically relevant security issue. One purpose of this parallel CVE list
is to provide that missing technical and prioritization information.

The original goal of the AL-KERNEL project was to prioritize Linux kernel
CVE analysis automatically before manual review. The system can also help
identify non-security issues that may be suitable for automatic closure.

This report was generated by AL-KERNEL, an AI-assisted Linux kernel
vulnerability analysis system developed by Alexander Larkin.

The first analysis stage combines an autonomous classifier with additional
LLM-based analysis. The autonomous classifier runs locally on a CPU and is
based on a backpropagation neural network. Together, these mechanisms
produce a technical vulnerability description, identify likely weakness
types, estimate CVSS severity, and provide input for ActionableScore.

Two CVSS estimates are retained because incomplete kernel vulnerability
information often permits more than one defensible interpretation:

  Conservative CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:N
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:L
  The Best / paranoid CVSS score: 4.5

The conservative vector represents a lower-impact interpretation.
The Best/paranoid vector intentionally represents a plausible upper-bound
interpretation and should not automatically be treated as demonstrated
real-world impact.

CVSS may also need to be adjusted for a particular Linux deployment,
because actual reachability, privileges, enabled kernel configuration,
hardware, namespaces, exposed device nodes, and other environmental
conditions can differ significantly between systems.

A separate ActionableScore mechanism evaluates practical remediation
urgency. Its analysis may include reachability, attack prerequisites,
subsystem exposure, memory-corruption characteristics, denial-of-service
reliability, and possible confidentiality, integrity, or
privilege-escalation impact.

  Conservative ActionableScore: 3
  Paranoid ActionableScore: 4

The final base severity is taken directly from the second tab-separated
field of the AL-KERNEL classification result. ActionableScore does not
replace or independently override that final AL-KERNEL decision, and
if ActionableScore adjusted impact level of ALKERNEL, then you would see
self-readable flags above like INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7.

For an AL-KERNEL result of MODERATE, this report uses the following
additional presentation split:

  ActionableScore below 5   -> MODERATE REGULAR
  ActionableScore 5 or more -> MODERATE 7.0

The distinction between MODERATE REGULAR and MODERATE 7.0 makes it
possible to identify Moderate issues that should receive manual analysis
and fixes before lower-priority MODERATE REGULAR issues. In many cases,
MODERATE REGULAR fixes may wait for a later rebase or routine update.

There is one override in which MODERATE REGULAR becomes MODERATE 7.0
even when the ActionableScore is below 5. When the AL-KERNEL result
contains the KPANIC flag, a MODERATE result is always presented as
MODERATE 7.0. The KPANIC flag selected with few regexps without
usage of AI at all, so it helps to detect cases when Kernel Crash happens
and similar (to filter False-Negative results from the LLM usage).

KPANIC indicates that a reliable kernel crash, kernel panic, or similarly
serious kernel availability impact was identified by the classification
workflow.

AL-KERNEL base severity for this report: MODERATE
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): MODERATE REGULAR

These results are intended to support engineering triage. They are
machine-generated estimates, and cases marked for manual review should
be validated by a human security engineer before final disposition.
For more info read docs linked from here: https://kernelcve.org/
(and you can submit you own patch there to generate such a report
for non-existant CVE-id yet).

Note that in many cases this AI tool selects higher severity, than
real is (means you can expect Importants instead of Moderate 7.0 or
Moderates 7.0 instead of regular Moderates). If you see such cases,
please use reply email interface to add additional manual analyses
info to this particular CVE.
And please, please, let me know when you see Lows instead of Importants
or Important instead of Low (because particular for such cases I
need to tune this AI tool to make it better for this one and next similar).
My contact email for such notifications is alexanjelausa@gmail.com (and both
send reply to CVE record itself too and see "reply" button below for howto reply).